Concern over the potential adverse health effects of chemically contaminated groundwater has existed for many years. In the present studies bone marrow parameters were monitored in female B6C3F1 mice exposed to a chemical mixture in drinking water for 17 days or up to 32 weeks. The mixture consisted of 25 common groundwater contaminants frequently found near toxic waste dumps, as determined by EPA surveys. The purpose of these experiments were to compare the hematopoisis among the different time courses of exposure and to determine whether residual damage could be demonstrated by subsequent radiations at a time when hematopoietic stem cells sustained injury or nearly recovered.